A quick and dirty option would be to use an API to pass the orders and loop through all the accounts doing that (i.e. one file with the list of orders, one with the list of accounts and a small program to pass the orders).

treat it like a hedgefund(limited partnership)
think of your account as a corporation.
assign a number shares to your account's netliquidating value(equity).
let your friends buy a given number of shares at a given price.
take your fee out and treat it as a cost to the fund that lowers the equity.

Repeat this process as you take in new money, or when u distribute profits/return funds.